doc: fix undefined reference for X509Certificate and format markdown

Author: haramj

doc/api/tls.md

@@ -2323,9 +2323,9 @@ added:
     **Default:** `"default"`.
   * `as` {string} The format of returned certificates. One of:
     * `"buffer"` (default): Returns an array of certificate data as `Buffer` objects.
-    * `"x509"`: Returns an array of [`X509Certificate`][] instances.
+    * `"x509"`: Returns an array of \[`X509Certificate`]\[] instances.
 
-* Returns: {Array.<Buffer|X509Certificate>}
+* Returns: {Array.\<Buffer|X509Certificate>}
   An array of certificates in the specified format.
 
 * `"default"`: return the CA certificates that will be used by the Node.js TLS clients by default.
@@ -2465,6 +2465,7 @@ added:
 [Session Resumption]: #session-resumption
 [Stream]: stream.md#stream
 [TLS recommendations]: https://wiki.mozilla.org/Security/Server_Side_TLS
+[X509Certificate]: https://nodejs.org/api/crypto.html#class-x509certificate
 [`'newSession'`]: #event-newsession
 [`'resumeSession'`]: #event-resumesession
 [`'secureConnect'`]: #event-secureconnect